The present disclosure generally relates to corrosion and, more specifically, to methods and systems for suppressing corrosion of titanium and titanium alloy surfaces.
Corrosive environments comprising an acid can cause severe corrosion damage to many types of metal surfaces. As used herein, the term “corrosion” and grammatical variants thereof refer to any reaction between a metal surface and its surrounding environment that causes a deterioration or change in the metal surface's properties or morphology. Examples of corrosion damage to a metal surface include, but are not limited to, rusting, metal dissolution or erosion, pitting, peeling, blistering, patina formation, cracking, embrittlement, and any combination thereof.
Acidic fluids are frequently utilized in the course of various commercial processes, such as when conducting various treatment operations in a subterranean wellbore. Corrosion-induced damage of metal surfaces during such processes can be highly undesirable. Corrosion of metal surfaces within a wellbore, such as tubulars and tools, for example, can be highly undesirable due to the difficulty, cost, and production downtime associated with replacing or repairing such components. In many instances, elevated temperatures within subterranean formations can also dramatically accelerate downhole corrosion rates. Regardless of its location and origin, corrosion-induced damage of a metal surface can represent a significant safety and/or environmental concern due to potential well failure issues.
Metal surfaces in fluid communication with a wellbore can likewise be susceptible to corrosion and its undesirable effects. Outside the wellbore, corrosion can occur prior to or during introduction of an acidic subterranean treatment fluid to the wellbore, during or following production of a spent or partially spent acidic subterranean treatment fluid, or any combination thereof. Metal components of surface well assemblies and systems, pipelines, and production facilities can be particularly susceptible in this regard. In subsea wellbores, a subsea riser structure extending from the wellbore (e.g., via a blowout preventer) to a platform or vessel on the ocean's surface or just below the ocean's surface can similarly be susceptible to corrosion during production of a partially spent acidic subterranean treatment fluid from the wellbore. The risk of corrosion to various components of a wellbore system can be so significant in some instances that exclusion of potentially corrosive agents may be warranted, possibly limiting the realm of subterranean treatment operations that are available to a well operator.
Organic corrosion inhibitors may be used to mitigate the corrosive effects of some mineral and organic acids, but not all, and numerous limitations exist. Certain metals are also more susceptible to the effects of corrosion than are others, and successful corrosion inhibitor strategies for one metal do not necessarily work for another. As used herein, the terms “inhibit,” “inhibitor,” “inhibition” and other grammatical forms thereof refer to the lessening of the tendency of a phenomenon to occur and/or the degree to which that phenomenon occurs. The terms “suppress,” “suppression” and other grammatical forms thereof may be used equivalently herein. The terms “inhibit” and equivalents thereof do not imply any particular extent or amount of inhibition or suppression unless otherwise specified herein.
Hydrofluoric acid- and acidic fluoride-containing fluids can be especially corrosive toward certain types of sensitive metal surfaces, such as those containing titanium or a titanium alloy. Titanium and titanium alloys are lightweight, strong and resistant to most formation fluids and a great number of common subterranean treatment fluids, including those containing organic acids and/or mineral acids such as hydrochloric acid. However, titanium and titanium alloys are especially prone to corrosion by even modest quantities of hydrofluoric acid or fluoride ions at pH values of about 7 or less. Moreover, conventional organic corrosion inhibitors are not especially effective for protecting titanium and titanium alloys against corrosion promoted by hydrofluoric acid. Without being bound by any theory or mechanism, it is believed that the extreme reactivity of titanium toward these types of fluids is due to removal of a passivating surface oxide by hydrofluoric acid. The extreme sensitivity of titanium and titanium alloys to hydrofluoric acid and acidic fluoride ions can preclude the use of hydrofluoric acid in various situations where this acid might otherwise be desirable. For example, titanium and titanium alloys are frequently present in expansion or stress joints of subsea riser structures and other components of wellbore systems, which can make stimulation operations very difficult to conduct in deepwater wellbores and other wellbores containing a siliceous material. Similar issues may be encountered in other industrial processes in which hydrofluoric acid or acidic fluoride ions come into contact with titanium-containing components.
Although inhibited, corrosion-resistant titanium alloys (e.g., Ti Grade 29 alloy, which is inhibited by small amounts of ruthenium, or Ti Grade 7 alloy, which is inhibited by small amounts of palladium) can display a decreased propensity toward corrosion in the presence of hydrofluoric acid or acidic fluoride ions compared to pristine titanium or uninhibited alloys (e.g., commercially pure titanium, CP-Ti), corrosion is often still an issue. Moreover, cost and sourcing of inhibited titanium alloys can be problematic, especially for large-scale operations.